1. Field of the Invention
The present invention relates to a photovoltaic device composed of a non-single-crystalline semiconductor material of a silicon type, and a process for producing the photovoltaic device. In particular, the present invention relates to an inexpensive solar cell exhibiting a high photoelectric conversion efficiency and a process for producing it. The present invention relates also to a zinc oxide thin film, and a process for producing it.
2. Related Background Art
In recent years, the demand for solar cells is growing as power generation equipment. In particular, photoelectric power generation is attracting attention which is conducted by solar cell modules of about 3 kW mounted on a roof of a private house and which is connected to a commercial power supplying system, whereby electric power is soled to or brought from the commercial system. A governmental subsidy system has already been enforced. Under such circumstances, the photoelectric power generation has disadvantages that the power generation cost is too high to compete the commercial power generation, and that the generation capacity is low to meet the power demand. To offset the above disadvantages, the solar cell is required to have a high photoelectric conversion efficiency, stable power generation ability for a long term of about 20 years or more, a high power generation capacity, and a low cost/power performance. At present, the material for the solar cell includes crystalline silicon (c-Si), polycrystalline silicon (poly-Si), amorphous silicon (a-Si), GaAs, and CdS. Of these, the amorphous silicon solar cell is advantageous in production performance and the cost/power performance. After the Kobe-Osaka-Awaji Great Earthquake Disaster, weight reduction of the roof material comes to be demanded. In this respect, light-weight amorphous silicon solar cell is advantageous. Further, the amorphous silicon solar cell is advantageous in installation on a curved surface. However, the amorphous solar cell does not achieve the high photoelectric conversion efficiency of the crystalline silicon solar cell at the moment, and is naturally deteriorated in photoelectric conversion efficiency by exposure to intense light.
Therefore, improvements of non-single-crystalline silicon type solar cells are widely investigated as below. For example, for amorphous silicon type materials, trials are being made for improving light collection efficiency by narrowing the bandgap by addition of Ge or Sn to the i-type layer in an amount ranging from 1% to 50%, or for raising the open circuit voltage by broadening the bandgap by addition of C, N, O, or the like in an amount ranging from 0.1% to 10%. Other trials are being made for collecting broader range of light by stacking an element having a higher open circuit voltage at a light introducing side and an element having a lower open circuit voltage at a back side. With the stack type solar cell, the thickness of the i-type layer is tried to be made smaller to retard the photo-deterioration. For example, a photoelectric conversion efficiency of 9.5% after photo-deterioration was achieved with a solar cell having constitution of a-Si/a-SiGe/a-SGe, or a-Si/a-Si/a-SiGe by S. Guha, J. Yang: Technical Digest of 7th International Photovoltaic Science and Engineering Conference Nov. 1993, NAGOYA JAPAN, p43 xe2x80x9cProgress in Multijunction Amorphous Silicon Alloy-Based Solar Cells and Modulesxe2x80x9d. A photoelectric conversion efficiency 10.2% after photo-deterioration of was achieved with a solar cell having constitution of a-SiC/a-SiGe/a-SiGe by K. Nomoto, Y. Yamamoto: Technical Digest of 7th International Photovoltaic Science and Engineering Conference Nov. 1993, NAGOYA JAPAN, p43 xe2x80x9cProgress in Multijunction Amorphous Silicon Alloy-Based Solar Cells and Modulesxe2x80x9d. xe2x80x9ca-Si Alloy Three-Stacked Solar Cells with High Stabilized-Efficiencyxe2x80x9d.
An attempt was made to lower the power cost by forming an a-Si layer and a-SiGe layer by microwave plasma CVD at a higher deposition rate by K. Saito, I. Kajita: Journal of Non-Crystalline Solids 146-166 (1993) p689-692 xe2x80x9cHigh efficiency a-Si:H alloy cell deposited at high deposition ratexe2x80x9d. According to this report, a photoelectric conversion efficiency of 11.6% was achieved with a constitution of a-Si/a-SiGe/a-SiGe by use of a-Si formed at a deposition rate of 75 A/sec and a-SiGe formed at a deposition rate of 100 A/sec.
A plasma CVD apparatus is disclosed which forms continuously semiconductor layers of different conduction types by a roll-to-roll system in Japanese Patent Application Laid-Open No. 05-121331. This apparatus has a plurality of deposition chambers, and a belt-like flexible substrate is arranged along the path so as to pass through the deposition chambers successively. The substrate is delivered in its length direction while a semiconductor layer of a desired conduction type is formed in each of the respective deposition chambers, thereby continuously producing a photovoltaic device having a p-i-n junction. The above disclosed apparatus employs a gas gate which prevents diffusion of the source gas for introducing valence electron controlling agent into the semiconductor layer to another deposition chamber and thereby prevents contamination of another semiconductor layer. Specifically, the deposition chambers are separated by a slit-shaped separation path where a sweeping gas such as Ar, H2 and He is introduced to prevent mutual diffusion of the source gases, whereby a desired p-i-n junction is formed. This roll-to-roll system for thin film formation improves remarkably the productivity of photovoltaic devices having a stacked structure.
A transparent electroconductive layer having a surface of a projection-recess structure (texture structure) is known to improve light collection efficiency. For example, Preprint of 51th Applied Physics Society Meeting p747 (1990 Autumn) 29p-MF-2 xe2x80x9cOptical Confinement Effect in a-SiGe Solar Cell on Stainless Steel Substratexe2x80x9d; and Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p387, 1987 disclose improvement of short-circuiting photoelectric current by forming a back reflection layer composed of Ag and a transparent layer composed of zinc oxide in a suitable surface texture structure. T. Tiedje, et al.: Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) p1423, and H. Deckman, et al.: Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) p1425 disclose improvement of photoelectric conversion efficiency by forming a back electrode into a projection-recess shape (texture structure) having a size approximate to light wavelength for scattering light to scatter long wavelength light which has not been absorbed in the semiconductor layer and lengthen the optical path in the semiconductor layer, thereby raising sensitivity of the photovoltaic device to respect with the long wavelength light to increase short-circuit photoelectric current.
Zinc oxide is more resistant to plasma than tin oxide and indium oxide, and when zinc oxide is exposed to plasma containing hydrogen, it is not reduced by hydrogen. When a semiconductor layer composed of amorphous silicon is formed on zinc oxide by plasma CVD, zinc oxide is positively used as a transparent electroconductive layer.
Japanese Patent Application Laid-Open No. 60-84888 (Energy Conversion Devices) discloses a technique for decrease of electric current passing through defective regions in a semiconductor layer by interposition of a transparent electroconductive layer between a back electrode and a semiconductor layer.
Japanese Patent Application Laid-Open No. 7-23775, and Masanobu Izaki, Takasi Omi: Journal of Electrochemical Soc. Vol.143, No.3 xe2x80x9cElectrolyte Optimization for Cathodic Growth of Zinc Oxide Filmsxe2x80x9d discloses electrochemical deposition of a transparent zinc oxide thin film by applying electric current between counter electrodes immersed in an aqueous zinc nitrate solution. This method requires neither an expensive vacuum apparatus nor an expensive target, thereby remarkably reducing the production cost of the zinc oxide. This method can be employed for deposition on a substrate of a large area and therefore is promising for a large-area photovoltaic device like solar cells.
As understood from the aforementioned disclosures, zinc oxide is desirable for use as the transparent electroconductive layer having a texture structure on the surface. The known process for producing zinc oxide thin film includes vacuum deposition, sputtering, ion-plating, and CVD. These methods employs an expensive vacuum apparatus, and the vapor deposition source is expensive, and does not exhibit sufficient optical confinement effect in the optical wavelength range from 600 nm to 1000 nm. Other methods include wet processes such as a spray-pyrolysis method, and a sol-gel method. In the wet processes, the substrate is required to be heated to a temperature ranging from 300xc2x0 C. to 800xc2x0 C., so that the useful substrates are limited. In spite of such many investigations, known photovoltaic devices do not satisfy all of the properties of high photoconductivity, high durability, high production yield, and low cost.
An object of the present invention is to provide a photovoltaic device which has solved the above problems.
Specifically, another object of the present invention is to provide a photovoltaic device by forming a transparent electroconductive layer composed of zinc oxide on an electroconductive substrate with sufficient adhesion without abnormal growth portion with suitable roughness of the surface to give sufficient optical confinement of the photovoltaic device.
Still another object of the present invention is to provide a flexible solar cell of light weight capable of generating electric power for a long term at a low power-generation cost.
A further object of the present invention is to provide a method of photovoltaic power generation by using a solar cell module constituted of the photovoltaic devices which is mounted on a roof of a private house.
A still further object of the present invention is to provide night illumination apparatuses in parks and roads, guide lamps, and indoor ventilation apparatuses, separated from commercial power lines.
In order to solve the above problems, the present invention provides a photovoltaic device comprising a back reflection layer, a zinc oxide layer, and a semiconductor layer stacked in this order on a substrate, wherein the zinc oxide layer contains a carbohydrate.
The content of the carbohydrate in the zinc oxide layer ranges preferably from 1 xcexcg/cm3 to 100 mg/cm3. The carbohydrate incorporated into the zinc oxide layer in the photovoltaic device of the present invention remarkably improves adhesion of the zinc oxide layer to an adjacent back reflection layer or an adjacent subbing layer such as an intermediate layer in comparison with conventional photovoltaic devices. Further the carbohydrate incorporated into the zinc oxide suitably controls the growth of the zinc oxide crystals to give a suitable surface roughness, thereby causing optical confinement or light scattering in the wavelength range from 600 to 1000 nm to obtain the excellent properties of the photovoltaic device of the present invention.
The aforementioned back reflection layer may be formed from gold, silver, aluminum, or copper. When aluminum is employed for the back reflection layer, a transparent and electroconductive intermediate layer is preferably provided on the aluminum layer by sputtering or a like method.
The present invention provides a zinc oxide thin film containing a carbohydrate at a content ranging from 1 xcexcg/cm3 to 100 mg/cm3.
The present invention further provides a process for electrochemically forming a zinc oxide layer by applying electric current between an electroconductive substrate and a counter electrode immersed in an aqueous solution containing at least zinc ions and a carbohydrate. The present invention further provides a process for producing a photovoltaic device employing the above process for forming the zinc oxide layer. The cost of production by the electrochemical method of the present invention is about 1/100 times that of a sputtering method.
The potential difference between the electroconductive substrate and the counter electrode is controlled preferably in the range from 0.1 V to 30 V. Under this condition, the carbohydrate can be suitably and stably incorporated in the zinc oxide layer to significantly retard abnormal crystal growth and thereby improve the yield.
The aforementioned aqueous solution contains preferably at least zinc ions, a carbohydrate and nitrate ions; it contains at least zinc ions, a carbohydrate, ammonium ions and complex ions such as zinc ammonia complex ion (including ammine complex ions); or it contains at least zinc ions, a carbohydrate, and hydrogenzincate ions or zincate ions.